Kerberos Working Group M. Swift
Internet Draft University of WA
Document: draft-ietf-krb-wg-kerberos-referrals-00.txt J. Brezak
Category: Standards Track Microsoft
J. Trostle
Cisco Systems
K. Raeburn
MIT
February 2001
Generating KDC Referrals to locate Kerberos realms
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026 [1].
Internet-Drafts are working documents of the Internet Engineering
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1. Abstract
The draft documents a new method for a Kerberos Key Distribution
Center (KDC) to respond to client requests for kerberos tickets when
the client does not have detailed configuration information on the
realms of users or services. The KDC will handle requests for
principals in other realms by returning either a referral error or a
cross-realm TGT to another realm on the referral path. The clients
will use this referral information to reach the realm of the target
principal and then receive the ticket.
2. Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
this document are to be interpreted as described in RFC-2119 [2].
3. Introduction
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Current implementations of the Kerberos AS and TGS protocols, as
defined in RFC 1510 [3], use principal names constructed from a
known user or service name and realm. A service name is typically
constructed from a name of the service and the DNS host name of the
computer that is providing the service. Many existing deployments of
Kerberos use a single Kerberos realm where all users and services
would be using the same realm. However in an environment where there
are multiple trusted Kerberos realms, the client needs to be able to
determine what realm a particular user or service is in before
making an AS or TGS request. Traditionally this requires client
configuration to make this possible.
When having to deal with multiple trusted realms, users are forced
to know what realm they are in before they can obtain a ticket
granting ticket (TGT) with an AS request. However, in many cases the
user would like to use a more familiar name that is not directly
related to the realm of their Kerberos principal name. A good
example of this is an RFC-822 style email name. This document
describes a mechanism that would allow a user to specify a user
principal name that is an alias for the user's Kerberos principal
name. In practice this would be the name that the user specifies to
obtain a TGT from a Kerberos KDC. The user principal name no longer
has a direct relationship with the Kerberos principal or realm. Thus
the administrator is able to move the user's principal to other
realms without the user having to know that it happened.
Once a user has a TGT, they would like to be able to access services
in any trusted Kerberos realm. To do this requires that the client
be able to determine what realm the target service's host is in
before making the TGS request. Current implementations of Kerberos
typically have a table that maps DNS host names to corresponding
Kerberos realms. In order for this to work on the client, each
application canonicalizes the host name of the service by doing a
DNS lookup followed by a reverse lookup using the returned IP
address. The returned primary host name is then used in the
construction of the principal name for the target service. In order
for the correct realm to be added for the target host, the mapping
table [domain_to_realm] is consulted for the realm corresponding to
the DNS host name. The corresponding realm is then used to complete
the target service principal name.
This traditional mechanism requires that each client have very
detailed configuration information about the hosts that are
providing services and their corresponding realms. Having client
side configuration information can be very costly from an
administration point of view - especially if there are many realms
and computers in the environment.
Current implementations of Kerberos also have difficulty with
services on hosts that can have multiple host names (multi-homed
hosts). Traditionally, each host name would need to have a distinct
principal and a corresponding key. An extreme example of this would
be a Web server with multiple host names for each domain that it is
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supporting. Principal aliases allow multi-homed hosts to have a
single Kerberos principal (with a single key) that can have
identities for each distinct host name. This mechanism allows the
Kerberos client to request a service ticket for the distinct
hostname and allows the KDC to return a ticket for the single
principal that the host is using. This canonical principal name
allows the host to only have to manage a single key for all of the
identities that it supports. In addition, the client only needs to
know the realm of the canonical service name, not all of the
identities.
This draft proposes a solution for these problems and simplifies
administration by minimizing the configuration information needed on
each computer using Kerberos. Specifically it describes a mechanism
to allow the KDC to handle Canonicalization of names, provide for
principal aliases for users and services and provide a mechanism for
the KDC to determine the trusted realm authentication path by being
able to generate referrals to other realms in order to locate
principals.
To rectify these problems, this draft introduces three new kinds of
KDC referrals:
1. AS ticket referrals, in which the client doesn't know which realm
contains a user account.
2. TGS ticket referrals, in which the client doesn't know which
realm contains a server account.
3. Cross realm shortcut referrals, in which the KDC chooses the next
path on a referral chain
4. Realm Organization Model
This draft assumes that the world of principals is arranged on
multiple levels: the realm, the enterprise, and the world. A KDC may
issue tickets for any principal in its realm or cross-realm tickets
for realms with which it has a direct trust relationship. The KDC
also has access to a trusted name service that can resolve any name
from within its enterprise into a realm. This trusted name service
removes the need to use an untrusted DNS lookup for name resolution.
For example, consider the following configuration, where lines
indicate trust relationships:
MS.COM
/ \
/ \
OFFICE.MS.COM NT.MS.COM
In this configuration, all users in the MS.COM enterprise could have
a principal name such as alice@MS.COM, with the same realm portion.
In addition, servers at MS.COM should be able to have DNS host names
from any DNS domain independent of what Kerberos realm their
principal resides in.
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5. Principal Names
5.1 Service Principal Names
The standard Kerberos model in RFC 1510 [3] gives each Kerberos
principal a single name. However, if a service is reachable by
several addresses, it is useful for a principal to have multiple
names. Consider a service running on a multi-homed machine. Rather
than requiring a separate principal and password for each name it
exports, a single account with multiple names could be used.
Multiple names are also useful for services in that clients need not
perform DNS lookups to resolve a host name into a full DNS address.
Instead, the service may have a name for each of its supported host
names, including its IP address. Nonetheless, it is still convenient
for the service to not have to be aware of all these names. Thus a
new name may be added to DNS for a service by updating DNS and the
KDC database without having to notify the service. In addition, it
implies that these aliases are globally unique: they do not include
a specifier dictating what realm contains the principal. Thus, an
alias for a server is of the form "class/instance/name" and may be
transmitted as any name type.
5.2 Client Principal Names
Similarly, a client account may also have multiple principal names.
More useful, though, is a globally unique name that allows
unification of email and security principal names. For example, all
users at MS may have a client principal name of the form
"joe@MS.COM" even though the principals are contained in multiple
realms. This global name is again an alias for the true client
principal name, which is indicates what realm contains the
principal. Thus, accounts "alice" in the realm ntdev.MS.COM and
"bob" in office.MS.COM may logon as "alice@MS.COM" and "bob@MS.COM".
This requires a new client principal name type, as the AS-REQ
message only contains a single realm field, and the realm portion of
this name doesn't correspond to any Kerberos realm. Thus, the entire
name "alice@MS.COM" is transmitted in the client name field of the
AS-REQ message, with a name type of KRB-NT-ENTERPRISE-PRINCIPAL.
KRB-NT-ENTERPRISE-PRINCIPAL 10
5.3 Name Canonicalization
In order to support name aliases, the Kerberos client must
explicitly request the name-canonicalization KDC option (bit 15) in
the ticket flags for the TGS-REQ. This flag indicates to the KDC
that the client is prepared to receive a reply with a different
client or server principal name than the request. Thus, the
KDCOptions types is redefined as:
KDCOptions ::= BIT STRING {
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reserved(0),
forwardable(1),
forwarded(2),
proxiable(3),
proxy(4),
allow-postdate(5),
postdated(6),
unused7(7),
renewable(8),
unused9(9),
unused10(10),
unused11(11),
name-canonicalize(15),
renewable-ok(27),
enc-tkt-in-skey(28),
renew(30),
validate(31)
}
6. Client Referrals
The simplest form of ticket referral is for a user requesting a
ticket using an AS-REQ. In this case, the client machine will send
the AS request to a convenient trusted realm, either the realm of
the client machine or the realm of the client name. In the case of
the name Alice@MS.COM, the client may optimistically choose to send
the request to MS.COM.
The client will send the string "alice@MS.COM" in the client
principal name field using the KRB-NT-ENTERPRISE-PRINCIPAL name type
with the crealm set to MS.COM. The KDC will try to lookup the name
in its local account database. If the account is present in the
crealm of the request, it MUST return a KDC reply structure with the
appropriate ticket. If the account is not present in the crealm
specified in the request and the name-canonicalize flag in the
KDCoptions is set, the KDC will try to lookup the entire name,
Alice@MS.COM, using a name service. If this lookup is unsuccessful,
it MUST return the error KDC_ERR_C_PRINCIPAL_UNKNOWN. If the lookup
is successful, it MUST return an error KDC_ERR_WRONG_REALM (0x44)
and in the error message the cname and crealm field MUST contain the
client name and the true realm of the client. If the KDC contains
the account locally, it MUST return a normal ticket. The client name
and realm portions of the ticket and KDC reply message MUST be the
client's true name in the realm, not the globally unique name.
If the client receives a KDC_ERR_WRONG_REALM error, it will issue a
new AS request with the same client principal name used to generate
the first referral to the realm specified by the crealm field of the
kerberos error message from the first request. This request MUST
produce a valid AS response with a ticket for the canonical user
name. The ticket MUST also include the ticket extension containing
the TE-REFERRAL-DATA with the referred-names set to the name from
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the AS request. Any other error or referral will terminate the
request and result in a failed AS request.
7. Server Referrals
The server referral mechanism is a bit more complex than the client
referral mechanism. The primary problem is that the KDC must return
a referral ticket rather than an error message, so it will include
in the TGS response information about what realm contains the
service. This is done by returning information about the server name
in the pre-auth data field of the KDC reply.
If the KDC resolves the server principal name into a principal in
its realm, it may return a normal ticket. If the name-canonicalize
flag in the KDCoptions is not set, then the KDC MUST only look up
the name as a normal principal name. Otherwise, it MUST search all
aliases as well. The server principal name in both the ticket and
the KDC reply MUST be the true server principal name instead of one
of the aliases. This frees the application server from needing to
know about all its aliases.
If the name-canonicalize flag in the KDCoptions is set and the KDC
doesn't find the principal locally, the KDC can return a cross-realm
ticket granting ticket to the next hop on the trust path towards a
realm that may be able to resolve the principal name.
If the KDC can determine the service principal's realm, it can
return the server realm as ticket extension data. The ticket
extension MUST be encrypted using the session key from the ticket,
and the same etype as is used to protect the TGS reply body.
The data itself is an ASN.1 encoded structure containing the
server's realm, and if known, canonical principal name and alias
names. The first name in the sequence is the canonical principal
name.
TE-REFERRAL-INFO 20
TE-REFERRAL-DATA ::= SEQUENCE {
referred-server-realm[0] KERB-REALM
referred-names[1] SEQUENCE OF
PrincipalNames OPTIONAL
}
The client can use this information to request a chain of cross-
realm ticket granting tickets until it reaches the realm of the
server, and can then expect to receive a valid service ticket.
In order to facilitate cross-realm interoperability, a client SHOULD
NOT send short names in TGS requests to the KDC. A short name is
defined as a Kerberos name that includes a DNS name that is not
fully qualified. The client MAY use forward DNS lookups to obtain
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the long name that corresponds to the user entered short name (the
short name will be a prefix of the corresponding long name).
The client may use the referred-names field to tell if it already
has a ticket to the server in its ticket cache.
The client can use this information to request a chain of cross-
realm ticket granting tickets until it reaches the realm of the
server, and can then expect to receive a valid service ticket.
However an implementation should limit the number of referrals that
it processes to avoid infinite referral loops. A suggested limit is
5 referrals before giving up.
8. Cross Realm Routing
The current Kerberos protocol requires the client to explicitly
request a cross-realm TGT for each pair of realms on a referral
chain. As a result, the client machines need to be aware of the
trust hierarchy and of any short-cut trusts (those that aren't
parent-child trusts). This requires more configurations on the
client. Instead, the client should be able to request a TGT to the
target realm from each realm on the route. The KDC will determine
the best path for the client and return a cross-realm TGT. The
client has to be aware that a request for a cross-realm TGT may
return a TGT for a realm different from the one requested.
9. Security Considerations
The original Kerberos specification stated that the server principal
name in the KDC reply was the same as the server name in the
request. These protocol changes break that assumption, so the client
may be vulnerable to a denial of service attack by an attacker that
replays replies from previous requests. It can verify that the
request was one of its own by checking the client-address field or
authtime field, though, so the damage is limited and detectable.
For the AS exchange case, it is important that the logon mechanism
not trust a name that has not been used to authenticate the user.
For example, the name that the user enters as part of a logon
exchange may not be the name that the user authenticates as, given
that the KDC_ERR_WRONG_REALM error may have been returned. The
relevant Kerberos naming information for logon (if any), is the
client name and client realm in the service ticket targeted at the
workstation that was obtained using the user's initial TGT.
How the client name and client realm is mapped into a local account
for logon is a local matter, but the client logon mechanism MUST use
additional information such as the client realm and/or authorization
attributes from the service ticket presented to the workstation by
the user, when mapping the logon credentials to a local account on
the workstation.
10. Discussion
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This section contains issues and suggestions that need to be
incorporated into this draft. From Ken Raeburn [raeburn@mit.edu]:
1) No means to do name canonicalization if you're not
authenticating. Is it okay to require credentials in order to do
canonicalization? If so, how about this: Send a TGS_REQ for the
service name you have. If you get back a TGS_REP for a service,
great; pull out the name and throw out the credentials. If you
get back a TGS_REP for a TGT service, ask again in the specified
realm. If you get back a KRB_ERROR because policy prohibits you
from authenticating to that service, we can add to the
specification that the {realm,sname} in the KRB_ERROR must be the
canonical name, and the checksum must be used. As long as the
checksum is present, it's still a secure exchange with the KDC.
If we have to be able to do name canonicalization without any
sort of credentials, either client-side (tickets) or server-side
(tickets automatically acquired via service key), I think we just
lose. But maybe GSSAPI should be changed if that's the case.
2) Can't refer to another realm and specify a different service name
to give to that realm's KDC. The local KDC can tell you a
different service name or a different realm name, but not both.
This comes up in the "gnuftp.raeburn.org CNAME ftp.gnu.org" type
of case I've mentioned.
Except ... the KDC-REP structure includes padata and ticket
extensions fields that are extensible. We could add a required
value to one of them -- perhaps only in the case where you return
a TGT when not asked -- that contains signed information about
the principal name to ask for in the other realm. (It would have
to be required, otherwise a man-in-the-middle could make it go
away.) Signing would be done using the session key for the TGS.
3) Secure canonicalization of service name in AS_REQ. If the
response is an AS_REP, we need a way to tell that the altered
server name wasn't a result of a MITM attack on the AS_REQ
message. Again, the KDC-REP extensible fields could have a new
required value added when name canonicalization happens,
indicating what the original principal name (in the AS_REQ
message) was, and signed using the same key as protects the
AS_REP. If it doesn't match what the client requested, the
messages were altered in transit.
4) Client name needs referral to another realm, and server name
needs canonicalization of some sort. The above fixes wouldn't
work for this case, and I'm not even sure which KDC should be
doing the canonicalization anyways.
The other-principal-name datum would probably look something like:
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PrincipalAndNonce ::= SEQUENCE {
name[0] PrincipalName,
nonce[1] INTEGER -- copied from KDC_REQ
}
SignedPrincipal ::= SEQUENCE {
name-and-nonce[0] PrincipalAndNonce,
cksum[1] Checksum
}
{PA,TE}-ORIGINAL-SERVER-PRINCIPAL ::= SignedPrincipal
{PA,TE}-REMOTE-SERVER-PRINCIPAL ::= SignedPrincipal
with the checksum computed over the encoding of the 'name-and-nonce'
field, and appropriate PA- or TE- numbers assigned. I don't have a
strong opinion on whether it'd be a pa-data or ticket extension;
conceptually it seems like an abuse of either, but, well, I think
I'd rather abuse them than leave the facility both in and
inadequate.
The nonce is needed because multiple exchanges may be made with the
same key, and these extension fields aren't packed in with the other
encrypted data in the same response, so a MITM could pick apart
multiple messages and mix-and-match components. (In a TGS_REQ
exchange, a subsession key would help, but it's not required.)
The extension field would be required to prevent a MITM from
discarding the field from a response; a flag bit in a protected part
of the message (probably in 'flags' in EncKDCRepPart) could also let
us know of a cases where the information can be omitted, namely,
when no name change is done. Perhaps the bit should be set to
indicate that a name change *was* done, and clear if it wasn't,
making the no-change case more directly compatible with RFC1510.
11. References
1 Bradner, S., "The Internet Standards Process -- Revision 3", BCP
9, RFC 2026, October 1996.
2 Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997
3 Kohl, J., Neuman, C., "The Kerberos Network Authentication
Service (V5)", RFC 1510, September 1993
12. Author's Addresses
Michael Swift
University of Washington
Seattle, Washington
Email: mikesw@cs.washington.edu
John Brezak
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Microsoft
One Microsoft Way
Redmond, Washington
Email: jbrezak@Microsoft.com
Jonathan Trostle
Cisco Systems
170 W. Tasman Dr.
San Jose, CA 95134
Email: jtrostle@cisco.com
Kenneth Raeburn
Massachusetts Institute of Technology 77
Massachusetts Avenue
Cambridge, Massachusetts 02139
Email: raeburn@mit.edu
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